A cross member structure of a vehicle body includes a floor panel (2) provided below a vehicle interior in the vehicle body (10); side sills (3) that are provided on both sides of the floor panel (2) in a vehicle width direction and extend in a front-rear direction of the vehicle body (10); a cross member (4) that extends in the vehicle width direction and connects the side sills (3) to each other; and a reinforcement (5) that is provided between the cross member (4) and the floor panel (2) and extends in the vehicle width direction, wherein the cross member (4) includes a curved portion (41) that is curved to at least one side in a vertical direction of the vehicle body (10) outward from the reinforcement (5) in the vehicle width direction.